Contagious Equine Metritis

Contagious equine metritis (CEM) is a highly infectious venereal 

disease of horses. It is caused by the bacteria Taylorella 

equigenitalis. CEM spreads during breeding or through genital 

contact with contaminated objects. It does not affect other 

livestock or people. This disease could cause considerable 

economic losses to the equine industry if it became established 

in this country. That’s why the U.S. Department of Agriculture 

(USDA) works with State partners to protect our country’s 

horses against CEM. As a horse owner, breeder, or large animal 

veterinarian, you should learn the signs of this disease and put in 

place measures to protect your horses or horses under your care.

History and Distribution of CEM

The first cases of CEM were diagnosed in England in the 1970s. 

Since then, CEM has been diagnosed in horses in many countries 

around the world. The United States is currently considered 

free of CEM. USDA’s Animal and Plant Health Inspection Service 

maintains a list of CEM-affected countries on its website (www.

aphis.usda.gov, search “animal health status of regions”). The 

list includes Japan, South Africa, Korea, all European Union 

countries, and many other countries in Europe, including Great 

Britain and Northern Ireland. For more information on CEM 

reporting around the world, visit the World Animal Health 

Information System at wahis.woah.org. 

Controlling CEM in the United States

To help keep this disease out of the United States, USDA 

requires stallions and mares imported from CEM-affected 

countries to quarantine and test negative for CEM before they 

can enter the country. 

If a case of CEM is detected in this country, animal health officials 

identify, test, and treat all infected or exposed horses with 

disinfectant scrubs and antimicrobials to eliminate the bacteria 

T. equigenitalis. While the United States has had sporadic cases 

of CEM over the years due to the inadvertent importation of 

infected horses, we have successfully eradicated the disease 

from our equine population. The most widespread U.S. outbreak 

of CEM occurred in 2008. It took 2 years to eliminate the disease. 

After expensive and time-consuming testing of more than 1,000 

horses, USDA was able to regain the country’s CEM-free status. 

Signs of Infection

Horses infected with CEM can be difficult to identify. Stallions 

carry the bacteria without showing any signs at all and appear 

unable to clear infection without treatment. Carrier stallions can 

silently spread the disease to many other horses at a breeding 

facility, including other stallions, before CEM infection is 

suspected or diagnosed. Infection in mares may also be difficult 

to detect but is usually more noticeable. Mares may exhibit the 

following signs of infection: 

 A thick, milky, mucoid discharge 10 to 14 days after breeding 

to an infected stallion

 Returning to heat (estrus) prematurely

 Short-term infertility after breeding to an infected stallion and 

failing to conceive on the first or second estrus after exposure 

Unlike stallions, most affected mares will clear the infection 

without treatment within a few months. However, some mares 

will remain chronically infected, with or without clinical signs. 

These mares are capable of infecting other horses even while 

successfully carrying pregnancies. 

How CEM Spreads

T. equigenitalis, the bacteria that causes CEM, spreads in the 

following ways: 

 During live cover breeding

 Through artificial insemination with contaminated semen 

(chilled or frozen)

 Via contact with contaminated objects, including on hands 

or sheath cleaning and semen collection equipment 

Infected mares able to successfully carry a pregnancy may rarely 

transmit the disease to their foals.

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Testing for CEM

Private veterinarians should work with State or Federal animal 

health officials to test horses suspected of having CEM or that 

were exposed to a CEM-positive animal. CEM testing may also be 

necessary to meet requirements for exporting horses, semen, 

or embryos to another country, or as part of a routine breeding 

soundness examination. There are three types of official tests: 

bacterial culture, blood test, and test breeding.

Bacterial cultures test for growth of T. equigenitalis, the 

bacteria that cause CEM, in swab samples. Veterinarians 

collect swabs and send them to a USDA-approved laboratory. 

(www.aphis.usda.gov/animal_health/lab_info_services/

downloads/ApprovedLabs_CEM.pdf). The samples require 

special handling and must be processed within 48 hours of 

collection. It takes 7 days to complete the culture process. 

Typically, veterinarians will collect three sets of swabs for 

culturing during a 7- to 12-day period.

Blood testing by complement fixation test (CFT) detects an 

immune response to T. equigenitalis infection. It can help to 

determine if a horse has recently been exposed to the disease. 

This test can only be used in mares because stallions do not 

develop an immune response to infection.

Test breeding is required for stallions under certain 

circumstances in the United States, such as known exposure to 

T. equigenitalis or importation from CEM-affected countries. It 

is more sensitive than direct sampling and culture of the stallion. 

To test breed, a stallion is bred to two mares known to be CEMfree. Those mares are then tested for CEM. It takes five weeks and 

multiple tests on the mares to declare the stallion negative. Test 

breeding is not normally required to export stallions or semen to 

another country.

Polymerase chain reaction, or PCR, testing for T. equigenitalis is 

not currently recognized in the United States as an official test for 

CEM. USDA is currently evaluating this testing method. 

Treatment and Recovery

Infected mares and stallions can be treated using disinfectant 

scrubs and antimicrobials in a specific 5-day treatment protocol. 

Fertility appears to return to normal in recovered horses.

Prevention

You can prevent the spread of CEM by: 

 Conducting bacterial culture tests on breeding stallions prior 

to each breeding season 

 Testing any mares that exhibit clinical signs consistent with 

CEM 

 Using proper biosecurity measures, such as cleaning and disinfecting semen collection equipment between stallions 

 Keeping records on horse movements and semen shipments 

so they can be traced if a case of CEM is detected 

Reporting Suspicious Cases

If you suspect an animal may have CEM or any other foreign 

animal disease, immediately contact your State Animal Health 

Official (www.usaha.org/saho) or Federal animal health authority 

(www.aphis.usda.gov/animal_health/contacts/field-operationsdistricts.pdf).

Learn More

For more information on CEM, call USDA Veterinary Services at 

(970) 494-7391 or send an email to equine.health@usda.gov. 

You can also visit us online at www.aphis.usda.gov/aphis/

ourfocus/animalhealth.